This invention relates to methods and apparatus for determining the characteristics of an acoustic field. More particularly, this invention relates to the determination of the structure of complex sound fields within a duct of other structural enclosure through the use of holographic interferometry.
There are many situations in the field of acoustics wherein it is desirable or necessary to determine the structure of a complex sound field that is induced within a duct-like structure when that structure is acoustically excited by one or more sources of acoustic energy. For example, in the design of modern transport aircraft, a determination of the sound field existing within various portions of the aircraft is important in the design and optimization of acoustically treated structurel. In this regard, it is well-known that lightweight and efficient acoustic treatment systems are both necessary and desirable within the aircraft fuselage section so that the aircraft crew and passengers are not subjected to high noise levels. Further, acoustic treatment of various portions of an engine installation is often undertaken so that the noise propagating from the aircraft engines is minimized.
With respect to the design and optimization of such aircraft noise suppression systems and other situations in which it is desirable or necessary to determine the characteristics of a sound field within duct-like structure or structurally bounded enclosures of various other geometry, it is often advantageous to resolve the sound field into a plurality of spatial wave components or modes which, when linearly combined, provide a substantially complete description of the acoustic field. In this regard, those skilled in the art recognize that the discrete modal components that can exist within any particular structure are determined by the geometry of that structure and by mathematical boundary conditions that reflect various physical constrains (e.g., preserving the continuity of acoustic pressure at an interface between two media having different propagation constant or recognizing that the acoustic particle velocity is zero in a direction normal to a structural boundary). However, since practical situations such as the above-noted design of noise suppression apparatus for aircraft often involve structure of relatively complex geometry and usually involve a number of sources of acoustic energy that cannot be accurately modeled as point sources of spherical waves, a mathematical or theoretical derivation of the sound field that exists in such structure is generally not practical. Thus, a variety of various methods and apparatus have been proposed for accurate experimental determination of acoustic fields.
Often these prior art systems utilize an array of microphones or a single microphone that is moved about within the pressure field to supply electrical signals representative of the acoustic pressure at a plurality of positions within the structure of interest. Although such systems are capable of providing acoustic pressure information that can be analyzed to supply the desired information, several disadvantages and drawbacks are encountered. For example, measurement of the acoustic pressure with a single microphone is extremely time-consuming and since it is necessary to correlate the signal data obtained at the various microphone positions, measurement of even relatively simple time-varying fields is usually not possible. On the other hand, although the utilization of an array of microphones partially eliminates some of these problems, conventional data acquisition systems generally do not provide enough data channels for the simultaneous recording of sufficient pressure information to permit accurate and precise resolution of the pressure field. In this respect, the determination of amplitudes of the various acoustic pressure modes propagating in the inlet duct of a jet engine with an array of microphones would generally require more than one hundred microphones. Since conventional data acquisition systems do not provide for the simultaneous recording of such a large number of signals, repeated measurements are generally necessary and the problems associated with measurement with a single microphone are not completely eliminated. Further, although rather small microphones are available, the placement of one or more microphones in the interior region of structure such as a gas turbine inlet can disrupt or modify the acoustic environment and thereby cause measurement errors. In addition, in some situations it is not possible to place microphones in the structure under consideration without physically modifying that structure and rendering it useless for its intended purpose. For example, it may be necessary to cut openings through the walls of the structure in order to situate the microphones in the acoustic field.
As described in more detail hereinafter, the practice of this invention involves the application of interferometric holography as a means for measuring changes in pressure that occur along the boundary surfaces of a structure that contains or directs acoustic pressure variations. As is known in the interferometric holography art, when an optically reflective transducer diaphragm or other object that is subjected to time-varying displacement due to changes in fluid pressure or mechanical vibration is illuminated by a beam of coherent monochromatic light that is provided by a laser or other source and the light reflected from the diaphragm is combined with a portion of the original illumination, an optical interference pattern that is representative of the displacement of the illuminated object is produced. For example, U.S. Pat. No. 3,548,643 issued to Leith et al. describes a holographic vibration analysis method wherein a vibrating object is illuminated by a single pulse of monochromic coherent light with the pulse length being such that a relatively small displacement of the vibrating object takes place during the time in which the light impinges on the object. The light that is reflected from the object is directed to a photographic plate or other detector which also receives a reference beam that is derived from the laser illumination. The reference beam and the reflected object beam combine to form a diffraction pattern and, in particular, because of the movement of the object during the illumination interval, form an interference pattern that is representative of the displacement occurring during the period of illumination.
Another application of holographic interferometry is disclosed by Cindrich in U.S. Pat. No. 3,590,640 which discloses a sensor for measuring the pressure of a gas or liquid. The pressure sensor disclosed by Cindrich includes a cylindrical pressure chamber with one circular end wall thereof being formed by a diaphragm which deforms at the pressure within the chamber changes. The outer surface of the diaphragm is illuminated with an image of the diaphragm at rest which is reconstructed from a hologram. Thus, interference of the light from the reconstructed holographic image of the diaphragm and the light reflected from the actual diaphragm creates interference fringes whose number and arrangement are indicative of the pressure within the chamber.
The principles of holographic interferometry have also been applied in systems for the optical reconstruction of an acoustically illuminated object, such systems commonly being referred to as acoustical holography. In this respect, U.S. Pat. No. 3,919,881 issued to Metherell disclose a method and apparatus for acoustical holography that does not require an ultrasonic reference wave in order to create an interference pattern that includes both the amplitude and phase information of the signal reflected from an ultrasonically illuminated object. In particular, the Metherell patent discloses a system in which an object to be optically reconstructed is submersed in liquid and illuminated with ultrasonic energy so that wave energy that is reflected from the object impinges on a deformable, diffusely reflecting surface which forms one boundary wall of the tank that contains the illuminated object and the ultrasonic generator. The deformable surface is then illuminated by two relatively short pulses of coherent monochromatic light that are provided by a pulsed laser, with the to pulses of light energy being separated by one half the period of the ultrasonic signal utilized to acoustically illuminate the object. Further, the distance traversed by the second optical pulse is increased by one quarter of an optical wavelength relative to the distance travelled by the first optical pulse. The light energy reflected from the deformable surface during each illumination pulse is caused to impinge on and doubly expose a photographic plate that also receives an optical reference beam that is derived from the laser illumination to thereby form an interferogram. Since the two photographic exposures are separated by a period of time equal to one half a cycle of the acoustic energy, the two exposures result in an interferogram wherein the variations in the optical density of the photographic plate represent twice the actual displacement of the various regions of the deformable surface. Further, since the optical distance travelled by the second illuminating pulse is increased by one quarter of an optical wavelength, this pulse is effectively in phase quadrature with the first illuminating pulse and, as fully described in the Metherell patent, the optical density of the holographic interferometric record is linearly related to the displacement of the deformable surface. Once the interferogram has been formed in the manner disclosed by Metherell, the photographic plate is developed and illuminated with coherent monochromatic light to provide a second photographic record of the deformed surface. Upon development, the second photographic record can be used in a conventional holographic wavefront construction system to generate a visual image of the original object.
Although prior art such as the above-noted patents to Leith et al., Cindrich and Metherell demonstrate the application of holographic interferometry to certain physical situations involving objects that are subjected to vibration or deformation, the apparatus and techniques disclosed therein are not applicable to the previously mentioned situations in which it is necessary to determine the spatial characteristics of a complex sound field propagating through or contained in a structural enclosure. In this respect, and with particular reference to determining the sound field within an aircraft fuselage or portions of an aircraft engine installation such as the air inlet or exhaust systems, the structure being considered is often subjected to mechanical vibration as well as acoustic energy and the disclosed methods of interferometric holography do not provide for distinguishing between the interference pattern that results from the vibration and the interference pattern that results from the acoustic field. Further, present conventional pressure sensors or transducers that are utilized in forming holograms are not amenable to placement within structural enclosures such as the air inlets of gas turbine installations.
Accordingly, it is an object of this invention to provide a method and apparatus for convenient determination of the spatial characteristics of complex sound fields within various acoustically excited structural enclosures wherein little or no modification of the structural enclosure is necessary.
It is another object of this invention to provide a method and apparatus for determining the spatial characteristics of a complex sound field wherein holographic interferometry is employed to overcome the disadvantages encountered with previous measurement systems in which electroacoustic transducers are employed.
It is yet another object of this invention to provide a system for determining the spatial structure of sound fields wherein the structure enclosing or containing the sound field is also subjected to mechanical vibration.
Still further, it is an object of this invention to provide means for rapidly determining the sound field within structures such as an air inlet of an aircraft engine installation wherein such determination can be made with the engine in operation.